In the News & Views 'Ultracold gases: Shock cooling a universe' (Nature Physics 9, 605–606; 2013) by Martin W. Zwierlein the first sentence of the seventh paragraph should have read 'A topological defect can only form where at least three regions of different <Ψ> meet on a line, and only if the angle of <Ψ> changes by a non-zero multiple of 2π as one goes around that line — the marble makes a non-zero number of round trips around the Mexican hat'. This error was unfortunately introduced during the editorial process. Corrected after print 7 November 2013.